Conventionally, when a member to be tightened is tightened and fixed by screwing a bolt or a nut to a female screw, a nut, or a bolt, the following method was generally adopted to prevent an occurrence of loosening due to insufficient tightening force (tightening axial force), or to prevent damage to a member due to excessive tightening force. That is, the tightening rotational torque was controlled to a prescribed value by tightening a bolt or nut with a torque wrench, in order to maintain an appropriate tightening state with a prescribed tightening force.
However, there was no guarantee that the tightening axial force actually applied to a bolt conformed to the designed value, even when the tightening rotational torque was kept constant, due to the friction coefficient between a head portion of a bolt or a nut and a washer, the friction coefficient between the washer and a member to be tightened, the dispersion of the friction coefficient on the screw surface, and deposits such as dust or oil on each of the contacting surfaces. Thereby the reliability of the tightening state was low. It was also necessary to tighten with multitudes of bolts and nuts for maintaining safety, causing an increase in weight, and a vicious circle was generated requiring an increase in bolts and nuts to correspond to the weight increase, and there was a problem of the cost increase.
Generally, a spring washer is interposed between the male screw member and the female screw member that are mutually screwed together, and tightened and fixed, or a locknut is used, as loosening-preventing-means in conventional screw mechanisms.
However, the spring washer does not necessarily perform a sufficient loosening-preventing-effect, and the locknut cannot always be used, because it is necessary to protrude the male screw member from the female screw member for a large amount, in order to use the locknut. Even when these loosening-preventing-means were installed, it was possible to loosen by rotating the nut or bolt in the opposite direction with the same torque used in the tightening. There was a demand for loosening-preventing-means that disabled loosening once the tightening has been done, even when the same torque as in tightening was applied in the opposite direction, for a more reliable loosening-preventing-means.
For one of such loosening-preventing-means, those disclosed in Japanese Patent Laid-Open Publication No. Hei. 10-122223 is known, for example. A description of this disclosure follows, with reference to FIG. 22, FIG. 23A, and FIG. 23B. In a case of fixing a mounting member 53 to a female screw member 52, which has a female screw 52a formed, using a male screw member 51 such as a bolt having a threaded portion 51b, a pressing member 54 that is integrally coupled with a head portion 51a in the rotating direction, and a washer 55 that is integrally coupled with the mounting member 53 in the rotating direction, are interposed between the head portion 51a of the male screw member 51 and the mounting member 53.
A spiral-form cam face 56 that gradually heightens along the circumference in the counterclockwise direction, and a tier face 57 in an axial direction that connects both ends of the cam face 56 are formed on one end face of the washer 55. There is a dig-in-portion 58 formed on the other end face of the washer 55 that digs-in to the mounting member 53 and integrally couples therewith in the rotating direction. A mating groove 54a is formed on one end face of the pressing member 54, which integrally couples with the head portion 51 a in the rotating direction by mating therewith. And a spiral-form cam face 59 that face-contacts with the cam face 56, and a tier face 60 that connects both ends of the cam face 59 are formed on the other end face. The leads of the cam faces 56 and 59 are configured to be bigger than the screw leads of the screw members 51 and 52.
In the above-mentioned construction, the bolt 51 is inserted into the pressing member 54 and the washer 55, and the bolt 51 penetrates the mounting member 53 and screwed together with the female screw member 52, and tightened together firmly. This causes the bolt 51 and the pressing member 54 to integrally rotate with the head portion 51a and the groove 54a mating together, and the washer 55 also rotates through the interposition of the mating tier faces 57 and 60, and is pressed firmly to the mounting member 53 in the axial direction by the lead of the screw. The dig-in-portion 58 of the washer 55 digs into the mounting member 53, and the washer 55 and the mounting member 53 are integrally coupled in the rotating direction (particularly in the loosening rotating direction). In this state, the mounting member 53 is tightened together and fixed to the female screw member 52 by the bolt 51, through the interposition of the pressing member 54 and the washer 55.
When the bolt 51 and the female screw member 52 tries to rotate in a relative loosening direction in this state, the bolt 51 is moved in the axial direction through the mating of the cam faces 56 and 59, since the female screw member 52 is integrated in the rotating direction with the cam face 56 of the washer 55 through the interposition of the pressing member 53, and since the bolt 51 is integrated in the rotating direction with the cam face 59 of the pressing member 54. And because the leads of the cam faces 56 and 59 are bigger than the screw leads of the bolt 51 and the female screw member 52 screwed together, a strong tensile force in the axial direction is acted on the bolt 51, and prevents the relative rotation of the bolt 51 and the female screw member 52 in the loosening direction.
Furthermore, in Japanese Examined Utility Model Publication No. Sho. 37-3016 and Japanese Examined Utility Model Publication No. Sho. 37-31930, mechanisms are disclosed in which a pressing member 54 is integrated with a head portion 51a of a bolt 51, a washer 55 is in a ring shape, and notches or multitudes of protrusions are formed on the underside of the washer 55, as non-slip-means.
In order to achieve a desired loosening-preventing-effect with the above-mentioned construction, the cam faces 56 and 59 have to be processed with high precision so that a slip is surely generated between the cam faces 56 and 59, before a slip is generated between the bolt 51 and the pressing member 54, or between the washer 55 and the mounting member 53. However, it is especially difficult to process the bordering portions between the cam faces 56, 59, and the tier faces 57, 60, in high precision, and the above-mentioned effects cannot be achieved when there is unevenness or protrusions in this border portion. Hence, there was a problem that the cost became high due to the high precision processing, and not practical.
In a state just before the completion of the tightening in the above-mentioned construction, the bolt 51 and the pressing member 54 are integrated in the rotating direction, and this pressing member 54 and the washer 55 are mated together in the rotating direction through the interposition of the tier faces 60 and 57. Hence, when the bolt 51 is tightened and rotated, the washer 55 is required to rotate with the dig-in-portion 58 digging into the mounting member 53, and most of the rotating torque was required for the rotation of the washer 55, and did not contribute sufficiently to the tightening-torque of the washer 51. So there was a problem that not enough tightening force was achieved for the mounting member 53, in comparison to the applied rotation torque, and a shortage of the tightening force occurred.
On the other hand, in order to achieve a loosening-preventing-effect, there was a need to generate a slip between the cam faces 56 and 59, by making the bolt 51 and the pressing member 54 integrated in the rotating direction, as noted above, and making the pressing member 54 simultaneously rotated when the bolt 51 rotates in the loosening direction, and rotate relative to the washer 55 which is integrated in the rotating direction with the mounting member 53. Hence, it was necessary to be constructed in such a manner that a slip did not generate between the bolt 51 and the pressing member 54, before a slip was generated between the cam faces 56 and 59. And there was a problem that it was difficult to fulfill both of these requirements.
When a large rotating torque is added to the bolt 51 in order to achieve the necessary tightening force, a large load is acted on the tier faces 60 and 57 of the pressing member 54 and the washer 55. There is fear of an excessive stress-concentration generating at the corner to thereby generate a crack, and fear of damaging the pressing member 54 or the washer 55. An expensive material or a heat-treatment is required to prevent such an occurrence, and there was a problem that the cost increases.
In case of tightening and fixing a mounting member with low anti-crack characteristics, such as glass or ceramics, it was not possible to tighten firmly because there was fear of generating a crack due to the tightening. And there was a problem that it was very difficult to achieve a loosening-prevention effect.
In light of the above-mentioned conventional problems, an object of the present invention is to provide a tightening method for screws, and a screw mechanism, which surely controls the tightening force with precision, and achieve a highly reliable tightening state.
Another object of the present invention is to provide a loosening-preventing apparatus for the screw mechanism, which achieves a positive non-loosening-effect with practical processing costs, and which surely tightens and fixes even a mounting member with low anti-crack characteristics.